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The part I received for some upcoming Christmas playing of Eight Carol Accompaniments for Brass a5, Arranged by David Willcocks has the text "(C for C1)" and "(C for C2)" at the top of some of the pieces. I initially assumed it must be some kind of octave indicator, but I can't see that it has any effect on the notation. So, what does it mean?

Possibly choir 1 and 2? I just played a piece that at bone 1 and 2 for each of two choirs. really cool sound as the choirs did a echo/restate sort of thing.

You'd need to look at the score to tell though.